1. Field of the Invention
The present invention relates to a single-mode optical fiber applied to a transmission line for optical communications or the like and, in particular, to a dispersion-shifted fiber suitable for wavelength division multiplexing (WDM) transmission.
2. Related Background Art
Conventionally, in optical communication systems employing single-mode optical fibers as their transmission lines, light in the wavelength band of 1.3 .mu.m or 1.55 .mu.m has often been utilized as signal light for communications. Recently, from the viewpoint of reducing transmission loss in transmission lines, the light in the 1.55-.mu.m wavelength band has been in use more and more. The single-modeoptical fiber applied to such a transmission line for light in the wavelength band of 1.55 .mu.m (hereinafter referred to as 1.55-.mu.m single-mode optical fiber) is designed so as to nullify its wavelength dispersion (phenomenon in which pulse wave broadens because the propagating speed of light varies depending on its wavelength) for light in the wavelength band of 1.55 .mu.m (thus yielding a dispersion-shifted fiber having a zero-dispersion wavelength of 1.55 .mu.m). As such a dispersion-shifted fiber, for example,
Japanese Patent Publication No. 3-18161 discloses a dispersion-shifted fiber having a refractive index profile of a dual-shape-core structure, whose core region is constituted by an inner core and an outer core having a refractive index lower than that of the inner core. Also, Japanese Patent Application Laid-Open No. 63-43107 and Japanese Patent Application Laid-Open No. 2-141704 disclose a dispersion-shifted fiber having a refractive index profile of a depressed cladding/dual-shape-core structure, whose cladding region is constituted by an inner cladding and an outer cladding having a refractive index greater than that of the inner cladding. Further, V. A. Bhagavatula et al., OFC' 95 Technical Digest, Paper ThH1, 1995, and P. Nouchi et al., ECOC' 96, Paper MoB.3.2, 1996 disclose a dispersion-shifted fiber having a refractive index profile of a ring-shaped core structure.
Recently, on the other hand, since long-haul transmission has become possible with the advent of wavelength division multiplexing (WDM) transmission and optical amplifiers, there have been proposed, in order to avoid nonlinear optical effects, dispersion-shifted fibers employing a refractive index profile of the above-mentioned dual-shape-core structure or depressed cladding/dual-shape-core structure, whose zero-dispersion wavelength is shifted to the shorter wavelength side or longer wavelength side than the center wavelength of signal light (Japanese Patent Application Laid-Open No. 7-168046 and U.S. Pat. No. 5,483,612). Here, the nonlinear optical effects refer to phenomena in which signal light pulses are distorted in proportion to density of light intensity or the like due to nonlinear phenomena such as four-wavemixing (FWM), self-phase modulation (SPM), cross-phase modulation (XPM), or the like. Transmission speed and relaying intervals in repeating transmission systems are restricted by the nonlinear optical effects.
Japanese Patent Application Laid-Open No. 8-248251 proposes an optical fiber having a configuration which suppresses the occurrence of the above-mentioned nonlinear optical phenomena, which may be generated when light having a high power is incident on the optical fiber, thereby reducing the distortion in optical signals caused by these nonlinear optical phenomena. Such an optical fiber has a refractive index profile whose effective core cross-sectional area A.sub.eff is designed to be greater than about 70 .mu.m.sup.2.
Here, as disclosed in Japanese Patent Application Laid-Open No. 8-248251, the effective core cross-sectional area A.sub.eff is given by the following expression (1): ##EQU1##
wherein E is an electric field accompanying propagated light, and r is a radial distance from the core center.
On the other hand, dispersion slope is defined by the gradient of a graph indicating a dispersion characteristic in a predetermined wavelength band.